Rigorous procedure followed by shape keys to merge models
Let's take an example. A plane projected onto some steps using shrinkwrap modifier:

We apply as shape keys.
And now, here is how the plane moves for several values for the shape key:

As you can see, this is linear.
If a vertex goes from X when shape key is 0 to Y when 1, then its position is:
X + &lt;shape key value&gt; * (Y - X)

Note that we take here shrinkwrap example, but shape keys go the same whatever the way they are produced from.